In a semiconductor device, as typified by a diode, a MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), and an IGBT (Insulated Gate Bipolar Transistor), a termination region for enhancing breakdown voltage performance is formed so as to surround a region (active region) that functions as an active element.
The breakdown voltage of a semiconductor device includes a reverse breakdown voltage of a diode and an off breakdown voltage of a transistor. Either of them is a breakdown voltage in a case where the semiconductor device does not function as the active element, and a depletion layer spreading in the semiconductor maintains the breakdown voltage performance.
Here, in a semiconductor device having no termination region provided in an active region, only a low breakdown voltage is obtained, because the spread of a depletion layer is insufficient and moreover, due to geometric effects, an electric field concentrates in the boundary (normally having a columnar shape) of the depletion layer against the active region side. Accordingly, a configuration is adopted in which an implantation layer having the conductive type opposite to that of a semiconductor substrate is formed adjacent to a peripheral portion of the active region, to thereby expanding the depletion layer so that the concentration of an electric field in the peripheral portion of the active region is relieved and thus the breakdown voltage of the semiconductor device is increased. This configuration for increasing the breakdown voltage, which is provided outside the active region, is called a termination region.
For example, the breakdown voltage of a PN junction made by an N-type semiconductor substrate and a high-concentration P-type implantation layer is lowered since an electric field concentrates in a columnar junction of the peripheral portion of the high-concentration P-type implantation layer. Accordingly, when the low-concentration P-type implantation layer is formed adjacent to the peripheral portion of the high-concentration P-type implantation layer, the depletion layer spreads over both the N-type semiconductor substrate (drift layer) and the low-concentration P-type implantation layer. Thus, the breakdown voltage is increased. This low-concentration P-type implantation layer is generally called a RESURF (RESURF: Reduced Surface Field) layer or a JTE (Junction Termination Extension). Such a termination region structure is called a RESURF structure.
In the RESURF structure, the depletion layer spreads over both the drift layer and the RESURF layer, and thereby high breakdown voltage performance is obtained. The spread of the depletion layer depends on the equilibrium of the amount of space charges. Therefore, optimum implantation conditions (implantation conditions by which the highest breakdown voltage is obtained) in the RESURF layer are determined not by the concentration but by the implantation amount (dose amount). In a case where the implantation amount throughout the RESURF layer is uniform, an optimum implantation amount (implantation surface density) is, irrespective of the drift layer concentration, about 1×1012 cm−2 in a case of a Si (silicon) substrate, and about 1×1013 cm−2 (when the rate of activation is 100%) in a case of a 4H—SiC (silicon carbide) substrate. These are called RESURF conditions.
However, the RESURF structure has a demerit that, in order to obtain high breakdown voltage performance, the electric field intensity in the outermost periphery of the RESURF layer inevitably increases. As a result, an increase in the breakdown voltage is limited to the breakdown voltage in the outermost periphery of the RESURF layer, and a risk of occurrence of thermal destruction and flashover due to a short-circuit current caused in the breakdown increases.
Such concentration of an electric field in the outermost periphery of the RESURF layer is caused mainly by bias in the distribution of space charges in the depletion layer. More specifically, in the outermost periphery of the RESURF layer, cancellation is not successfully caused in the vector sum of electric fields from space charges (in a case of P-type, acceptor ions with negative charge) of the RESURF layer and space charges (in a case of N-type, donor ions with positive charge) of the drift layer. From a qualitative viewpoint, the depth of the depletion layer in the drift layer gradually decreases in a direction from the active region toward the outside of the RESURF layer. Accordingly, by progressively reducing the implantation amount in the RESURF layer toward the outside as disclosed in Non-Patent Document 1, the concentration of an electric field in the RESURF layer can be avoided.
As a result of the avoidance of the concentration of an electric field in the RESURF layer, an increased margin is obtained with respect to the breakdown electric field. Thus, under condition that the termination region has the same width, a higher breakdown voltage is obtained. From another viewpoint, the width of the termination region required for obtaining a certain breakdown voltage can be reduced. Additionally, progressively reducing the implantation amount in the RESURF layer toward the outside can improve the resistance to interface charges and an external electric field.
In a method for forming the RESURF layer disclosed in Non-Patent Document 1, an impurity is implanted by using a mask with the percentage of openings in the mask being varied, and then thermally diffused to thereby uniformize the concentration. However, such a method requires a mask pattern that is finer than the thermal diffusion length of the impurity. Therefore, this method cannot be applied when a thick-film resist mask is needed, such as when MeV (Mega-Electron-Volt) ion implantation is performed. Also, this method cannot be used for a semiconductor material, such as SiC, in which thermal diffusion of an impurity is extremely small.
Practically, therefore, a RESURF structure in which the implantation amount in the RESURF layer decreases stepwise toward the outside is adopted, as in Patent Documents 1 and 2. In this case, an electric field concentrates not only in the peripheral portion of the active region but also a boundary between RESURF layers having different implantation amounts and in the outermost periphery of the RESURF layer. However, under the condition that the same bias voltage is applied, the concentration of an electric field is largely relieved as compared with the RESURF layer having a single implantation amount.